The liquid crystals used in the field effect liquid crystal display are of the nematic type with positive dielectric anisotropy. Such materials are placed in a thin film between transparent electrodes in the form of a 90.degree. twist from the inner surface on one electrode to the inner surface on the other electrode. When an electric field is applied, the liquid crystals align themselves with the field. As is well known, this characteristic enables the liquid crystal to either block or transmit light in a display having polarized end plates depending upon the relative polarization of the plates.
One system employed as the liquid crystal in a twist nematic display is the Schiff base system of ##STR2## Schiff base systems are not stable in the presence of water which attacks the --CH.dbd.N-- link breaking the molecule into two starting materials. The display degradation appears as a decrease in operating temperature range, a significant power increase, and finally a blacking out of the display. Thus, Schiff base materials must be processed in an inert atmosphere and the assembled display must have a true hermetic seal.
Another disadvantage of a Schiff base material is that it is sensitive to ultra-violet light. Degradation from this source is manifest in an increase in power consumption. Because of the aforementioned disadvantages, other compositions have been tested to replace the Schiff base materials in such displays.
Recently, liquid crystal systems of the mixed ester type have been developed for use in a field effect display. These systems are preferable to the aforementioned Schiff base ones because they are more stable in the presence of water or ultra-violet light. A twist characteristic is imposed by placing the liquid crystal between the inner surface of two electrodes which have been treated to impart surfaces aligned at 90.degree. to each other. The liquid crystal adjacent to such surfaces orient along the 90.degree. alignment and form a natural twist.
One type display of mixed ester liquid crystal is set forth in a paper entitled "Design Considerations for Positive Dielectric Nematic Mixtures Suitable for Display Applications", presented at the Fifth International Liquid Crystal Conference -- Stockholm, Sweden, June 17-21, 1974. That paper discloses a mixture of six different p,p'-phenyl benzoate esters of the following formula:
Table I ______________________________________ p,p'-Phenyl Benzoate Esters ##STR3## Compound X Y ______________________________________ I CH.sub.3 O n-C.sub.5 H.sub.11 II n-C.sub.5 H.sub.11 O n-C.sub.5 H.sub.11 III n-C.sub.5 H.sub.11 CN IV n-C.sub.7 H.sub.15 " V n-C.sub.5 H.sub.11 O " VI n-C.sub.7 H.sub.15 O " ______________________________________
The above paper states the composition has a nematic temperature range of from -7.degree. to 58.degree..
Two requirements of a liquid crystal composition are a nematic temperature range of sufficient breadth for operation in varying climates, e.g., at least from 0.degree. C to 50.degree. C and a positive anisotropy or .DELTA..epsilon.&gt; 0. The cyano materials in the above table contribute to positive anisotropy while the alkyl benzoates contribute to a wide temperature range.
A major problem of liquid crystal ester mixtures is that they have a relatively slow response time to return to the chiral state after the electric field is removed (termed "decay time"). The decay time is particularly slow at relatively high voltages, e.g., 3 volts or more.